1. Field of the Invention
The present invention relates generally to a remote monitoring system. It relates in particular to a monitoring system comprising one or more sensors, which utilize L-C (inductance-capacitance) or L-C-R (inductance-capacitance-resistance) resonant circuits, in combination with an interrogation means, to monitor a variety of properties, including strain, temperature, pressure, identification, performance, chemical phase transition (such as melting and state-of-cure), fluid level, wear, rotation rate, location and proximity. The system eliminates the need for physical connection to a power source (i.e., no lead wires) or to data acquisition equipment, and allows for multiple measurements using a single acquisition channel. Additionally, it does not require that the sensors be in proximity to any form of acquisition hardware and it facilitates use of a portable handheld interrogation unit.
2. Description of the Related Art
A magnetic field response sensor is a passive inductor-capacitive circuit designed to change correspondingly with a change in the physical state that the sensor measures. Use of inductors and capacitors to form resonant circuits is established in the literature. See, for example, D. Halliday and R. Resnick, Fundamental of Physics, 2nd Edition, Wiley, New York, pp. 624–634 or similar basic physics or electronics texts. Wireless measurement acquisition systems that use existing sensors physically connected to a power source, microprocessor and transmitters are described in Woodard, S. E., Coffey, N. C., Gonzalez, G. A., Taylor, B. D., Brett, R. R., Woodman, K. L., Weathered, B. W. and Rollins, C. H., “Development and Flight Testing of an Adaptable Vehicle Health-Monitoring Architecture,” Journal of Aircraft, Vol 1, No. 3, May–June 2004, pp 531–539. A method of acquiring measurements without the need for physical connection to a power source is the use of radio frequency identification (RFID) tags. This method relies on the use of radio-frequency integrated circuits functionally coupled to sensors. Representative of patents covering RFID tags is U.S. Pat. No. 5,420,757. An example of a system for interrogating fluid level is that presented by Kochin et al. in U.S. Pat. No. 6,335,690, which teaches a preferred separation distance between the sensor and the interrogator of less than 3.5 cm. U.S. Pat. No. 6,111,520 (Allen) and Fonseca, M. A., English, J. M., Arx, M. V. Allen, M. G., “High Temperature Characterization of Ceramic Pressure Sensors,”Proceeding of 1999 IEEE MEMS Workshop, pp 146–149 discuss several methods of magnetic field response sensor interrogation having the sensors within the perimeter of the antenna used for interrogation. Planar or laminar designs of L-C circuits include rectangular inductors (e.g., U.S. Pat. No. 6,025,735), spiral inductors (e.g., U.S. Pat. No. 6,111,520), parallel place capacitors (e.g. U.S. Pat. No. 6,335,690) and interdigitated capacitors (e.g., see K. G. Ong and C. A. Gaines, Smart Materials Structure, (9) 2000; 421–428).